Minimum Resolvable Temperature Difference Measurement Research Articles

Multiscale Deblurred Feature Extraction Network for Automatic Four-Rod Target Detection in MRTD Measuring Process of Thermal Imagers.

The minimum resolvable temperature difference (MRTD) at which a four-rod target can be resolved is a critical parameter used to assess the comprehensive performance of thermal imaging systems, which is important for technological innovation in military and other fields. Recently, there have been some attempts to use an automatic objective approach based on deep learning to take the place of the classical manual subjective MRTD measurement approach, which is strongly affected by the psychological subjective factors of the experimenter and is limited in accuracy and speed. However, the scale variability of four-rod targets and the low pixels of infrared thermal cameras have turned out to be a challenging problem for automatic MRTD measurement. We propose a multiscale deblurred feature extraction network (MDF-Net), a backbone based on a yolov5 neural network, in an attempt to solve the aforementioned problem. We first present a global attention mechanism (GAM) attention module to represent strong images of the four-rod targets. Next, a Rep VGG module is introduced to decrease the blur. Our experiments show that the proposed method achieves the desired effect and state-of-the-art detection results, which innovatively improve the accuracy of four-rod target detection to 82.3% and thus make it possible for the thermal imagers to see further and to respond faster and more accurately.

Objective evaluation method for advance thermal imagers based on minimum resolvable temperature difference

Advancement in thermal imaging technology has improved the day and night surveillance capability of electro-optical system. Thermal imager detects the object of interest by sensing the temperature difference. The performance evaluation of these thermal imagers plays a vital role prior to effective deployment. Key performance parameter of thermal imaging system is minimum resolvable temperature difference (MRTD), as it is able to determine the overall system performance involving a human operator. MRTD is capable of defining the minimum temperature difference required to resolve the bar target pattern of defined spatial frequencies and provides the true estimate of the range performance of a thermal imaging system. In conventional methods, 4-bar target of defined resolution is exposed to the minimum resolvable condition to compute MRTD values. Presently, subjective MRTD measurement employs an operator to observe the minimum resolvable condition, and therefore, measurement suffers from poor repeatability. During large volume production, this practice becomes a tedious procedure to evaluate large numbers of thermal imaging sights. To ease the performance evaluation procedure, an objective MRTD measurement method for the evaluation of the thermal imaging system has been presented. In the present work, objective MRTD measurement method for advance thermal imager by computing the modulation transfer function and noise equivalent temperature difference has been presented. Experiments have been conducted to measure subjective and objective MRTDs. Results for both subjective and objective MRTD measurements have been compared and presented. It is found that the proposed method is able to produce very close result within accuracy of more than 95% than that of conventional methods.

Virtual MRTD – an indirect method to measure MRTD of thermal imagers using computer simulation

Minimum resolvable temperature difference (MRTD) is considered as the most important parameter of thermal imagers. A new method of MRTD measurement without drawbacks of other methods is presented in this paper. Proposed MRTD measurement method coded as virtual MRTD is based on a three steps measurement concept using semi-automatic objective measurements and computer simulation. First, objective parameters of the tested thermal imager are measured. Second, software simulates this tested thermal imager and generates the image of 4-bar target of specified spatial frequency (size) and contrast (temperature difference). Third, a human observer analyses the images of the 4-bar target generated by the software on the screen of PC set and measures MRTD of the simulated thermal imager at specified set of spatial frequencies. The proposed method offers higher measurement speed, lower cost and typically better accuracy in comparison with the typical MRTD measurement method.

Development and validation of a quantitative model for the subjective and objective minimum resolvable temperature difference of thermal imaging systems

The performance evaluation of infrared (IR) imaging systems plays a vital role prior to effective deployment. The minimum resolvable temperature difference (MRTD) is the primary key parameter of an IR imaging system, because it describes the overall system performance including that of a human operator. MRTD defines the minimum temperature difference required to resolve the four-bar target of defined spatial frequencies and is used to estimate the recognition ranges of a thermal imaging system before deployment for field evaluation. Conventionally, MRTD measurements use the temperature difference of a four-bar target that is exposed to the minimum resolvable condition. In current practice, subjective MRTD measurement employs a human operator to observe the minimum resolvable condition and therefore suffers from poor repeatability. It is a tedious procedure to evaluate large numbers of thermal imaging sights during production. In this work, an accurate objective MRTD measurement method is described by measuring the modulation transfer function and the noise equivalent temperature difference (NETD) of the thermal imager. Experiments were carried out to measure both subjective and objective MRTD. The results of the subjective and objective MRTD measurements are presented along with a comparison of the results. It is observed that the percentage error in objective MRTD is very close to that from a conventional subjective MRTD measurement.

The Study on the MRTD Measurement System of Medical Infrared Thermal Imager Based on Sub-Pixel Edge Detection Algorithm

MRTD (minimum resolvable temperature difference) is an important parameter to evaluate the infrared thermal imager. MRTD is not only related to the spatial resolution of the system, but also to many factors such as optical system, detector array and so on. The measurement technique and principle of the medical infrared thermal imager’s MRTD is deeply analyzed, and the MRTD measurement model is established in this paper. The MRTD measurement system of infrared thermal imager based on CCD imaging technology is designed. CCD is used to carry on the interpretation of temperature difference in this system instead of the human eye, which can avoid the effect of some factors on the subjective image quality evaluation, such as the contrast, the human interference flaw and so on. The sub-pixel edge detection algorithm is used to deal with the four-bar target image, which can effectively suppress the interface of the random noise on four-bar target image. The experimental results show that the MRTD testing accuracy of the system can reach 0.055℃.

Equivalent-measurement evaluation method of minimum resolvable gas concentration

Currently, there is no standard method of evaluating the performance of the gas leak infrared imaging detection system. The evaluating criterions vary greatly and are deficient in aspects of completeness and accuracy, such as noise equivalent temperature difference, noise equivalent concentration path length, and minimum detectable leak rates. Minimum resolvable gas concentration (MRGC) is a latest proposed parameter for evaluating the performance of a passive gas leak infrared imaging detection system, which takes full advantage of the comprehensive evaluation capability of the temperature resolution and spatial resolution of the minimum resolvable temperature difference (MRTD) model. The MRGC takes into account the environmental and gas state parameters, the size of the gas plume and other factors which influence the MRGC measurement. However, the MRGC measurement system is complicated and many state parameters need to be controlled, especially the wide range and dedicated gas concentration meters are required. Therefore, the mathematical model of MRGC is derived and established. By comparing the principles and measurement methods of the performance parameters, MRGC and MRTD, a novel MRGC equivalent measurement evaluation method is proposed, on condition that the minimum resolvable radiation differences are equal. Using ethylene and ammonia as the target, the equivalently measured results of MRGC are obtained. The results show that the MRGC increases with the spatial frequency increasing and the smaller the temperature difference is between the gas and the background blackbody, the faster the MRGC increases. What is more, when the spatial frequency is fixed, MRGC increases with the gas temperature approaching to the background temperature. The background temperature varies asymptotically, which means that if the gas temperature equals the background temperature, the system cannot detect the gas four-bar pattern, no matter what the gas concentration is (here, the maximum gas concentration is 1 million ppm under normal pressure.). The directly measured and equivalently measured results of ethylene are in good agreement within errors of less than ±20%, and the maximum error is 18.26% at a spatial frequency of 0.214f0, which demonstrates the feasibility and effectiveness of the method. Because the equivalent measurement method only needs the traditional MRTD measurement results and the gas infrared spectrum database, it is simple and reliable, which is very significant for the study and application of the gas leak infrared imaging detection systems.

A case of non validity of MRTD concept

MRTD is considered as the most important parameter of thermal imagers. It is a function of a minimum temperature difference between the bars of the standard 4-bar target and the background required to resolve the thermal image of the bars by an observer versus spatial frequency of the target. Both theoretical models and experimental tests presented in available literature show MRTD as a monotonically increasing bijective mathematical function. This paper presents results of measurements of MRTD that contradict this common opinion. These results show that in rare cases MRTD can be a non-bijective function. Imagers having such MRTD function can fulfill formal requirements on MRTD but will perform poorly at real life conditions. Full Text: PDF References STANAG 4349, Measurement of minimum resolvable thermal difference (MRTD) of thermal cameras, 1995. ASTM standard E 1213-2002, Standard Test Method for Minimum Resolvable Temperature Difference for Thermal Imaging Systems G.C. Holst, Testing and evaluation of infrared imaging systems (JCD Publishing Company 2008). G.C. Holst, The Infrared & Electro-Optical Systems Handbook, Vol.3: Electro-Optical System Design, Analysis and Testing, Ch. 4, 206-207 (SPIE 1993). K. Chrzanowski, Testing Thermal Imagers - Practical Guide (Military University of Technology, Warsaw, Poland 2010). Night Vision Thermal Imaging Systems Performance Model, User?s Manual & Reference Guide, U.S Army Night Vision and Electronic Sensors Directorate, AMSEL-RD-NV-MS-SPMD, 2001. R.G. Driggers, A. Van Hodgkin, R.H. Vollmerhausen, P. O'Shea, "Minimum resolvable temperature difference measurements on undersampled imagers", Proc. SPIE 5076, 179 (2003). CrossRef MERX Website DirectLink Phare Program no PL0103.03.01 PL0108.19.01-L002, Purchase and installation of portable thermo-vision cameras, Phare Program No. PL0103.03.02 purchase of 5 surveillance units for helicopters, 2004.